This invention relates to a method of preparing a plastic substrate to improve the characteristics of its surface for the bonding thereto of a subsequently applied metal film, such as an electrolessly deposited metal film. The invention relates both to an improved laminate comprising a plastic substrate and metal film, as well as to the substrate itself, useful especially in the production of "additive" circuit boards for electrical and electronic equipment.
The method here disclosed is generally similar to that disclosed in U.S. Pat. Nos. 3,620,933 and 3,666,549, and is a modification of the procedure disclosed in copending application Ser. No. 303,369, filed Nov. 3, 1972 now abandoned. The procedure involves initially bonding a sacrificial oxidized metal foil by heat and pressure to a surface of the polymer substrate which is ultimately to be metal plated or otherwise metallized. The sacrificial metal foil is chemically stripped or dissolved from the surface of the substrate, after which the permanent metal film is deposited by known techniques. Application of the sacrificial oxidized foil to, and subsequent chemical stripping of it from, the plastic surface produces a microporous topography on the substrate surface that provides improved bonding characteristics for electrolessly plated metal film.
This invention is directed to the improvement in the foregoing procedures obtained by subjecting the surface of the plastic substrate to contact with a solution containing an organic silicon compound at some stage of the process subsequent to stripping of the sacrificial foil from the substrate. The improvement obtained by this step is evidenced not only in greater bonding or peel strength between the substrate and final metal film, but more especially in greater retention of such bonding strength after exposure of the laminate to elevated temperatures as, for example, soldering.
One of the main requirements of printed circuits in general, and additive circuits in particular, is that they must exhibit strong bonding of the final metal coating to the plastic substrate. The industry has adopted a minimum requirement of approximately 8 pounds per linear inch for adhesion between the conductor metal and the plastic substrate. Along with this is the further important requirement in a satisfactory printed circuit that the metal-to-polymer bond be stable at elevated temperatures up to around 500.degree. to 550.degree.F. Indeed, printed circuit boards as mass-produced today are subject to soldering operations at temperatures of this order, which operations are used to permanently mount the various electronic components of the electrical circuit on the board. Quite frequently, such soldering operation involves partially dipping and momentarily holding the circuit board in a bath of molten solder in order to effect soldering of all junctions in one step. This produces a substantial thermal shock to the laminate. It is imperative, therefore, that such soldering operation does not weaken the metal-to-polymer bond below the industry specification as to minimum bond strength.
It has been found during extensive experimentation that many occasions arise where printed circuit boards show excellent metal-to-polymer adhesion at room temperature, but that a dramatic decrease of deterioration results because of soldering or other high temperature exposure. It is accordingly an objective of the present invention to provide a method of producing consistently higher peel strengths between the final metal conductor film and its supporting plastic substrate, to be able to do this over a wider range of operating conditions in the preparation of a printed circuit, and thus provide greater tolerance for variables which inherently and unavoidably arise in the commercial production operations. It is especially an objective of this invention to materially improve thermal shock resistance of the final metal-plastic composite.
As noted briefly above, it has now been found that contacting the surface of the plastic substrate, at some stage subsequent to the step of chemically stripping the sacrificial foil but at least cotemporaneously with the step of electroless plating, with a solution of an organic silicon derivative, more especially one of the class comprising the amino alkanoxy substituted silanes, results in substantially improving the adhesion of the conductor metal to the substrate both before and after soldering operations. The silane can be applied from either aqueous or non-aqueous solution and may constitute a totally separate step in the process of effecting an electroless metal deposit on the substrate, or it may be incorporated in one of the activating and/or plating operations.
The mechanism by which the silicon derivative exerts its favorable reaction is not well understood. However it appears that the effect is one of slowing down the rate of deposition of the electroless metal, whereby there is more opportunity for the metal to fill in the microscopic crevices reproduced in the surface of the substrate by the sacrificial metal foil. It is postulated that the slowing down of the rate of deposition of electroless metal helps to prevent the bridging-over of the crevices by the metal before a firm root or anchor is established in the interstices by the deposition metal in the electroless deposition process. Whatever the explanation, the presence of trace amounts of silicon compounds on the surface during the metal deposition provides a definite improvement.
Silicon derivatives, and silanes in particular, have been widely used in industry to promote the physical properties of various "killed" polymers. Filled polymers are made by blending into the polymer during its molding operation particles of titanium dioxide, asbestos, sand and other solids. Silanes have been used to promote the wetting of the solid particles with the polymers during the molding operation, thereby avoiding de-wetting or separation of the plastic from the filler material during mechanical stress. There are numerous references in the literature relating to the use of silicon derivatives or reactive silanes in various interfacial applications. One excellent reference is entitled "Reactive Silanes as Adhesion Promoters to Hydrophilic Surfaces", by Edmond P. Plueddenann, published by Dow Corning Corporation, Midland, Michigan. Also, in the aforementioned pending application Ser. No. 303,369, the use of silanes during interfacial contacting of the sacrificial metal foil and plastic substrate is disclosed. It has also been proposed in the prior patent art, as for example in U.S. Pat. Nos. 3,475,186, 3,615,733 and 3,615,735, to incorporate silicon compounds directly in the electroless plating solution. So far as it is known, however, there has been no previous suggestion for using silane materials in combination with the sacrificial metal foil technique, where the silane is brought into contact with the substrate surface at any stage subsequent to the chemical stripping of the sacrificial foil. This procedure appears to afford certain advantages and improvements when used in place of, or in conjunction with, the silane treatment taught in the aforesaid pending application Ser. No. 303,369, or in the above-mentioned prior patents.
A wide variety of organic silicon derivatives is available but apparently not all are useful in the practice of the invention herein disclosed. The best results are obtained by the use of silane-type materials, and more particularly there is preferred for commercial practice of the invention a rather specific type of silane having the general formula: EQU R.Si(R.sub.1).sub.3
wherein R is a lower alkyl (up to 6 carbons) amino substituted radical, and R.sub.1 is a lower alkanoxy (up to 3 carbons) radical.
The following examples illustrate the invention but it is understood that these are not to be considered as comprehensive of all silicon derivatives useful in the practice of the invention.